1. Field of the Invention
The present invention relates to a color balance adjusting apparatus for adjusting color balance, especially white balance, which is for use in an imaging apparatus using a color image signal, such as a color video camera and a color television camera, and also to a method for adjusting the color balance.
2. Description of the Related Art
Conventionally, there are external and internal photometry methods for automatically adjusting the white balance of a color image.
In the external photometry method, the color temperature of an illuminant for illuminating an object is measured by an external sensor, and the gains of .image signals indicative of red and blue components are adjusted so that the ratio among red, green, and blue could become 1:1:1 in accordance with the difference between the measured color temperature and the color temperature of a standard illuminant.
In the internal photometry method, a color difference signal is used as color temperature data. The color difference signals R-Y and B-Y are integrated (averaged), and the gains of the red and blue component signals are controlled so that the integration results could be 0. In other words, under the assumption that the average value of so many pixels in the color image, in many cases, an achromatic color when the white balance is adjusted, the red and blue signals are adjusted so that the ratio of the integration results of the red, green, and blue signals could become 1:1:1.
However, in the external photometry method, if an illumination illuminating the entire scene including an object differs from light actually measured, specifically, if a bright outdoor object is imaged from the inside of a dark room, the white balance of the object may be adjusted on the basis of a color temperature differing from the actual one, since the color temperature of the illuminant used in the dark room is actually measured. As a result, a very unnaturally-colored image may be obtained.
On the other hand, in the internal photometry method, a problem will occur where the above-described assumption is not applicable. Integration of the entire image of an object with its background, such as a blue sky and a blue sea or of a red wall, provides a value indicating blue or red but not indicating an achromatic color. Therefore, if white balance adjustment is made to the object by means of the internal photometry method, the colors of the background may fade, or the colors of the object (man, etc. ) may be wrongly adjusted to become their complementary colors, providing an unnatural color tone.
As described above, in the conventional color balance adjustment, it is difficult to make correct color adjustment to all kinds of images.